How do you make a boat that a kid can build?

Use two plastic bottles empty bottles tightly sealed to form a catamaran shape and use balsa wood as a platform. Then attach handheld fans at the back of the platform. It will carry 4kg and it will go by itself. but if it doesn't need to go by itself u can take off the fans.

I got top marks in my class for simplicity and for the best working boat!!